The present invention relates to a valve timing control device for an internal combustion engine, which controls the timing of opening and closing an intake valve and/or an exhaust valve of the engine according to an engine operating condition.
Japanese Laid-Open Patent Publication No. 9-250309 discloses a valve timing control device that employs an electromagnetic brake to generate a braking force so as to actuate a rotational phase control mechanism and thereby control the rotational phase of an engine camshaft relative to an engine crankshaft.
The above electromagnetic brake has a braking member brought into friction contact with a rotary member of the rotational phase control mechanism so that the braking force is transferred from the electromagnet brake to the rotational phase control mechanism through the contact between the braking member and the rotary member. However, the braking member and the rotary member eventually wear at their contact surfaces even under normal use. Due to the frictional wear of these members, there arises a problem that the electromagnetic brake may not be able to impart a sufficient braking force to stably actuate the rotational phase control mechanism over the long-term service of the device.
In order to solve such a problem, it is conceivable to substitute a so-called xe2x80x9cnon-contact brakexe2x80x9d for the above electromagnetic brake. One example of the non-contact brake is an eddy-current brake disclosed in Japanese Laid-Open Patent Publication No. 2001-107712. The eddy-current brake develops a magnetic field across a rotating member upon energization of an electromagnetic coil, thereby inducing an eddy current in the rotating member. With the interaction of the eddy current with the magnetic field, the rotating member becomes subjected to a braking force. The braking force generated by the eddy-current brake depends on the amount of eddy current caused in the rotating member, i.e., the rotation speed of the rotating member relative to the magnetic field.
In the case of using such an eddy-current brake in the valve timing control device to actuate the rotational phase control mechanism, however, there arise another problem that, when the rotary member of the rotational phase control mechanism rotates at a low speed, the braking force cannot be generated sufficiently. The valve timing control device thus becomes slow in valve timing control response and, at the same time, cannot stably hold the adjusted rotational phase between the crankshaft and the camshaft under the influence of an alternating torque of the camshaft (i.e. a variable torque caused by the counterforce between a cam profile and a valve spring). For the reasons above, the actual use of the eddy-current brake in the valve timing control device to actuate the rotational phase control mechanism is difficult.
It is therefore an object of the present invention to provide a valve timing control device for an internal combustion engine, which employs a non-contact brake to actuate a rotational phase control mechanism for improvement of durability, without slowing down its valve timing control response and without failing to hold the rotational phase of an engine camshaft relative to an engine crankshaft.
According to one aspect of the invention, there is provided a valve timing control device for an internal combustion engine, comprising: a drive rotary member rotated by a crankshaft of the engine; a driven rotary member to rotate a camshaft of the engine upon rotation of the drive rotary member; a rotational phase control mechanism having an intermediate rotary member rotated relative to the drive and driven rotary members to cause relative rotation between the drive and driven rotary members; and a braking mechanism having a hysteresis brake that generates a braking force to cause the intermediate rotary member to rotate relative to the drive and driven rotary members.
According to another aspect of the invention, there is provided a valve timing control device for an internal combustion engine, comprising: a drive rotary member rotated by a crankshaft of the engine; a driven rotary member to rotate a camshaft of the engine; an intermediate rotary member rotated relative to the drive and driven rotary members to cause relative rotation between the drive and driven rotary members; biasing means for biasing the intermediate rotary member in a given rotation direction; a hysteresis brake that selectively applies a braking force to the intermediate rotary member to force the intermediate rotary member in a direction opposite to the given rotation direction.
The other objects and features of the invention will become understood from the following description with reference to the accompanying drawings.